LVDS signals typically have a magnitude in the order of one hundred millivolts and are used, for example, in twisted pairs connecting pieces of electronic equipment distant by tens of meters. In such a configuration, the grounds of the pieces of equipment, serving as a reference to the differential signals, generally do not have the same voltage level. As a result, the common-mode of a differential signal arriving at receiver equipment may be outside the range allowed by the equipment's differential signal processing circuitry. For example, the common-mode of the incoming signal may be established at a value in a range of about ten volts, while the processing circuitry is supplied between 0 and 3 V.
Various approaches may be implemented in the processing circuitry to bring the common-mode of the input signal within an allowed range. One approach is to use a divider bridge for each component of the differential input signal, whose division ratio is chosen according to the maximum amplitude of the common-mode and the operating range of the receiver circuit. For example, if the common-mode may vary within a range of ±5 V with respect to the ground of the receiver circuit, and the receiver circuit is supplied with 3 V, each of the divider bridges may be designed with a division ratio of 2, such that the common-mode of the incoming signal does not exceed 2.5 V. However, this approach divides the amplitude of the differential signal by four, typically reducing it to around 25 mV. This reduces the signal to noise ratio and makes the design constraints of the receiving circuit increasingly difficult.
In a common-mode compensation circuit disclosed in U.S. Pat. No. 7,088,166, each component of the incoming differential signal is applied to a respective input of a differential stage through a resistor. A device for regulating the common-mode is designed to pull through each of the resistors a current proportional to the sum of the components, this sum representing the common-mode of the incoming differential signal. Thus, when the common-mode of the incoming signal increases, the current increases in the two resistors simultaneously, which tends to offset the increasing voltage level at the inputs of the differential stage. More specifically, the sum of the components of the incoming signal, produced by a resistor bridge, is applied to the input branch of a current mirror. This mirror has two output branches with the same characteristics, each connected to a respective input of the differential stage.
Ideally, the proportionality factor is set so that the voltage drop caused by the current in each resistor is equal to the difference of the common-mode input signal relative to a reference level of the receiver circuit. In this case, the common-mode of the inputs of the differential stage remains at the reference level, regardless of the common-mode of the incoming signal. This setting is approximate in practice because it depends on many parameters such as the supply voltage, the temperature, and the process technology used to manufacture the circuit.